IRES
Chapter 4 Measurement units and conversion factors
Pending questions for the members of the Oslo Group:
A. Introduction
4.1. Energy products are measured in physical units by their weight or mass, volume, and energy. The measurement units that are specific to an energy product and are employed at the point of measurement of the energy flow are often referred to as “original” or “natural” units (IEA/Eurostat Manual page 19). Coal, for example, is generally measured by its mass or weight and crude oil by its volume. In cross-fuel tabulations such as the energy balances energy sources and commodities are also displayed in a “common unit” to allow comparison across sources. These “common” units are usually energy units and require the conversion of a quantity of a product from its original units through the application of an appropriate conversion factor.
4.2. When different units are used to measure a product, the compiler is left with the task of converting units which, in absence of specific information on the products (such as density, gravity and calorific value), may lead to different figures.
4.3. This chapter provides a review of the physical measurement units used for energy statistics, explains the concepts of original and common units, discusses the importance of conversion factors for the conversion from original to common units and presents standard conversion factors to use in absence of country- or region-specific conversion factors.
B. Measurement units
4.4. This section covers the “original” or “natural” units as well as the common units. It also makes reference to the International System of Units – often abbreviated as SI from the French “Système International d’Unités” – which is a modernized version of the metric system established by international agreement. It provides a logical and interconnected framework for all measurements in science, industry and commerce. The SI is built upon a foundation of seven base units plus two supplementary units. Multiples and sub-multiples are expressed in the decimal system. See Box 4.1for more details on SI.
4.5.
Standardization in the recording and presentation of quantities in original units is a primary task of an energy statistician before quantities can be analyzed or compared. (UN Manual F.44 page 11).
Box 4.1: International System of Units
1. Original units
4.6. As mentioned in para 4.1, original units are the units of measurement employed at the point of measurement of the product flow that are those best suited to its physical state (solid, liquid or gas) and that require the simplest measuring instruments (IEA/Eurostat Manual page 19). Typical examples are mass units (e.g. kilograms or tons) for solid fuels[1] and volume units (e.g. litres or cubic metres) for liquids and gases. The actual units used nationally vary according to country and local condition and reflect historical practice in the country, sometimes adapted to changing fuel supply conditions (IEA/Eurostat Manual page 177).
4.7. Electricity is measured in kilowatt-hour (kWh), an energy unit (although it is rather a unit of work) which allows one to perceive the electrical energy in terms of the time an appliance of a specified wattage takes to “consume” this energy. Heat quantities in steam flows are calculated from measurements of the pressure and temperature of the steam and may be expressed in calories or joules. Apart from the measurements to derive the heat content of steam, heat flows are rarely measured but inferred from the fuel used to produce them.
4.8. It should be noted that it may occur that, in questionnaires for the collection of energy statistics, data may be required to be reported in different units from the original/natural unit. For example, statistics on crude oil and oil products may be requested in a mass or weight basis since the heating value of oil products by weight displays less variation than the heating value by volume. Statistics on gases, as well as wastes, can be requested in terajoules or other energy unit in order to ensure comparability, since gases (and wastes) are usually defined on the basis of their production processes, rather than their chemical composition and different compositions of the same type of gas (or waste) entail different energy contents by volume. Collection of statistics on wastes in an energy unit is based on the measured or inferred heat output, as in energy statistics waste refers only to the portion used directly for heat raising.
Mass units
4.9. Solid fuels, such as coal and coke, are generally measured in mass units. The SI unit for mass is the kilograms (kg). 1000 kg correspond to one metric ton. Metric tons (tons) are most commonly used for example, to measure coal and their derivatives. One ton corresponds to 1000 kg. Other units of mass used by countries include: pound (0.4536 kg), short ton (907.185 kg) and long ton (1016.05 kg). Table 8 in the Annex presents the equivalent factors to convert different mass units.
Volume units
4.10. Volume units are original units for most liquid and gaseous, as well as for some traditional fuels. The SI unit for volume is the cubic metre which is equivalent to a kilolitre or one thousand litres. Other volume units include: the British or Imperial gallon (4.546 litres), United States gallon (3.785 litres), the barrel (159 litres) and the cubic feet, which is also used to measure volumes of gaseous fuels. Given the preference from oil markets for the barrel as a volume unit, the barrel per day is commonly used within the petroleum sector so as to allow direct data comparison across different time frequencies (e.g., monthly versus annual crude oil production). However, in principle other units of volume per time can be used for the same purpose. Table 9 in the Annex shows the equivalent factors to convert volume units.
Conversions between mass and volume - Specific gravity and density
4.11. Since liquid fuels can be measured by either weight or volume it is essential to be able to convert one into the other. This is accomplished by using the density of the liquid. Specific gravity is the ratio of the mass of a given volume of oil at 15°C to the mass of the same volume of water at that temperature. Density is the mass per unit volume.
Specific gravity= / Mass oil / Density= / massmass water / volume
COMMENT:
I suggest to split paragraph 4.11 into two separate paragraphs in this way:
Density. The relationship between mass and volume is called density and is defined as mass divided by volume. Since liquid fuels are measured either by their mass or volume it is essential to be able to convert one into the other. This is accomplished by using the density of the fuel.
Density
Specific Gravity is a dimensionless unit defined as the ratio of density of the fuel to the density of water at a specified temperature. This can also be expressed as the ratio of the mass of a given volume of fuel, for instance oil, at 15°C to the mass of the same volume of water at that temperature.
Specific gravity
4.12. When density is expressed in kilograms per litre, it is equivalent to the specific gravity. When using the SI or metric system, in order to calculate volume, mass is divided by the specific gravity or density. Vice versa, to obtain mass, volume is multiplied by the specific gravity or density. When using other measurement systems, one must consult tables of conversion factors to move between mass and volume measurements.
4.13. Another measure to express the gravity or density of liquid fuels is API gravity, a standard adopted by the American Petroleum Institute. API gravity is related to specific gravity by the following formula:
API gravity = / 141.5 / - 131.5specific gravity
4.14. Thus specific gravity and API gravity are inversely related. They are both useful in that specific gravity increases with energy content per unit volume (e.g. barrel), while API gravity increases with energy content per unit mass (e.g. ton).
Energy units
4.15. Energy, heat, work and power are four concepts that are often confused. If force is exerted on an object and moves it over a distance, work is done, heat is released (under anything other than unrealistically ideal conditions) and energy is transformed. Energy, heat and work are three facets of the same concept. Energy is the capacity to do (and often the result of doing) work. Heat can be a by-product of work, but is also a form of energy. For example, in an automobile with a full tank of gasoline, embodied in that gasoline is chemical energy with the ability to create heat (with the application of a spark) and to do work (the gasoline combustion powers the automobile over a distance).
4.16. The SI unit of energy, heat and work is the joule (J). Other units include: the kilogram calorie in the metric system, or kilocalorie, (kcal) or one of its multiples; the British thermal unit (Btu) or one of its multiples; and the kilowatt hour (kWh).
4.17. Power is the rate at which work is done (or heat released, or energy converted). Example: A given light bulb draws 100 joules of energy per second of electricity, and uses that electricity to emit light and heat (both forms of energy). The rate of one joule per second is called a watt. The light bulb, operating at 100 J/s, is drawing power of 100 Watts.
4.18. The joule is a precise measure of energy and work. It is defined as the work done when a constant force of 1 Newton is exerted on a body with mass of 1 gram to move it a distance of 1 metre. One joule of heat is approximately equal to one fourth of a calorie and one thousandth of a Btu. Common multiples of the joule are the megajoule, gigajoule, terajoule and petajoule.
4.19. The gram calorie is a precise measure of heat energy and is equal to the amount of heat required to raise the temperature of 1 gram of water at 14.5°C by 1 degree Celsius. It may also be referred to as an International Steam Table calorie (IT calorie). The kilocalorie and the teracalorie are its two multiples which find common usage in the measurement of energy commodities.
4.20. The British thermal unit is a precise measure of heat and is equal to the amount of heat required to raise the temperature of 1 pound of water at 60°F by 1 degree Fahrenheit. Its most used multiples are the therm (105 Btu) and the quad (1015 Btu).
4.21. The kilowatt hour is a precise measure of heat and work. It is the work equivalent to 1000 watts (joules per second) over a one hour period. Thus 1 kilowatt-hour equals 3.6x106 joules. Electricity is generally measured in kilowatt hour.
2. Common units
4.22. As mentioned before, the original units in which energy sources and commodities are most naturally measured vary (e.g. tons, barrels, kilowatt hours, therm, calories, joules, cubic metres), thus quantity of energy sources and commodities are generally converted into a common unit to allow, for example, comparisons of fuel quantities and estimate efficiencies. The conversion from different units to a common unit requires some conversion factors for each product.
4.23. The energy unit in the International System of Units is the joule which is very commonly used in energy statistics as a common unit. Other energy units are also used such as: the ton of oil equivalent (toe) (41.868 gigajoules), the Gigawatt-hour (GWh), the British thermal unit (Btu) (1055.1 joules) and its derived units – therm (1015 Btu) and quad (105 Btu) and the teracalorie (4.205 joules).
4.24. In the past, when coal was the principal commercial fuel, the ton of coal equivalent (tce) was commonly used. However, with the increasing importance of oil, it has been replaced by the ton of oil equivalent. Table 10 in Annex shows the conversion equivalents between the common units.
C. Calorific values
[the standard factors displayed in the tables are from the UN manual F. 44. They need to be reviewed and discussed]
GENERAL COMMENT:
We disagree with the use of the words “standard” or “default” calorific values. We suggest using “mean calorific values” or “proxy calorific values”, which reflects the fact that several calorific values are possible.
We think that two paragraphs on the importance and the meaning of mean calorific values should be considered:
"Calorific values of a specific energy product can be determined in laboratories using specific techniques. The calorific value thus obtained is valid for the analyzed specimen and can vary from specimen to specimen within a specific mine or field. Therefore, any determined value may be a mean value for mines, fields, regions or countries. Many countries have not determined their own mean calorific values and search this information elsewhere. Therefore tables of proxy calorific values are presented here.
Energy statistics and energy balances are compiled by countries on different detail levels, i.e. there is a also a difference between detailed calorific values and the proxy calorific values utilized on their level of detail. Transparency with respect to which calorific values that is used is essential. "